1. Technical Field
The present invention relates to an optical disk drive and, more particularly, to a tracking servo method employing a tracking error signal and a focus servo method employing a focus error signal.
2. Related Art
In relation to a tracking servo apparatus that lets; for instance, a beam spot, follow a target track, a hitherto known configuration includes supplying a tracking error signal to low frequency compensation means and high frequency compensation means, letting the low frequency compensation means make a correction to a low frequency component of the tracking error signal and the high frequency compensation means make a correction to a high frequency component of the same, adding the thus-corrected signals to each other, and supplying a resultant signal to drive means.
A disclosure provided by JP 62-8338 A includes inputting a tracking error signal into a high frequency equalizer circuit that makes a correction to a frequency characteristic of a high frequency component and also into a low frequency equalizer circuit that makes a correction to a frequency characteristic of a low frequency component and superposing the tracking error signal subjected to a low frequency correction on the tracking error signal subjected to a high frequency correction, thereby driving a tracking actuator.
Further, a disclosure provided by JP 2000-113473 A includes separating a servo filter that receives an input of a tracking error signal into high frequency compensation means and low frequency compensation means; adding together an output from the high frequency compensation means and another output from the low frequency compensation means, to thus generate a tracking drive signal; and subjecting the tracking drive signal to half wave rectification by means of a limit voltage that is generated from an average of the tracking drive signal and an offset voltage given by a d.c. power source, thereby driving a tracking actuator.
FIG. 7 shows a configuration of an optical disk drive described in connection with JP 2000-113473 A. An optical disk 100 is rotationally driven by rotational means 102. A light beam originating from an optical pickup 103 is collected by an objective lens, thereby generating a light spot on the optical disk 100. The optical pickup 103 is driven by a tracking actuator 114 in a radial direction of the optical disk. Optical information detection means 104 detects information about the light beam reflected by the optical disk 100. Off-track signal generation means 111 subjects an output from the optical information detection means 104 to envelope detection, thereby binarizing the output. Tracking error detection means 105 detects a tracking error signal that is a signal commensurate with an amount of deviation between the light spot and a track center of the optical disk. Track cross signal generation means 112 binarizes a tracking error signal. Direction detection means 108 detects a direction of relative movement between the light spot and the track by means of an output from the off-track signal generation means 111 and an output from the track cross signal generation means 112.
High frequency compensation means 106 enhances a high frequency component of the tracking error signal. Low frequency compensation means 107 enhances a low frequency component of the tracking error signal. A rectifier 116 is a rectifier equipped with average value processing means 115, a half wave rectifier 109, and a DC power source 110. Drive means 113 generates drive force from a signal output from the half wave rectifier 109 and drives the tracking actuator 114.
A tracking drive signal that is a result of addition of an output from the high frequency compensation means 106 to an output from the low frequency compensation means 107 performed by an adder 130 is supplied to the half wave rectifier 109 and also to average value processing means 115. The average value processing means 115 determines an average of the tracking drive signal and outputs the low frequency component from which the high frequency component has been eliminated. The thus-output average value and an offset voltage given by the DC power source 110 are added together by the adder 131, to thus generate a limit voltage. The thus-generated limit voltage is supplied to the half wave rectifier 109. The half wave rectifier 109 receives, as inputs, an output from the direction detection means 108, the tracking drive signal, and the limit voltage generated by the adder 131; switches between polarities to be subjected to half wave rectification according to the output from the direction detection means 108; and compares the tracking drive signal with the output from the adder 131, to thus perform limit processing. Half wave rectification is thereby performed with reference to the average value of the tracking drive signal.
Limit processing of the tracking drive signal is effective as a method for reducing unwanted current consumption of the tracking actuator without involvement of a reduction in servo frequency.
However, when there is a defect in an optical disk that exceeds the limit voltage; for instance, a local surface run-out or a decenter, the servo fails to perform tracking. A residual servo track eventually becomes large, which in turn deteriorates quality of a regenerated signal during data regeneration. Therefore, it becomes necessary to retry regeneration operation by reducing the number of rotations of an optical disk.
In the meantime, when a defect, such as that mentioned above, exists in the optical disk, a retry cannot be performed during data recording operation, whereupon a write error immediately occurs. In particular, in a write once optical disk, such as a CD-R and a DVD-R, a retry of writing operation is impossible to perform; therefore, a write error immediately occurs at that point in time. This also applies to a case where a focus drive signal is subjected to limit processing.
Accordingly, there has been a desire for an optical disk drive that can pursue power saving by reducing unwanted current consumption of a tracking actuator or a focus actuator and simultaneously assure recording quality even in a data recording period.